Conventionally, field instruments such as differential pressure/pressure transmitters, electromagnetic flow meters, positioners and the like that use two-wire transmission paths have been equipped with a CPU, and in addition to the primary measurement and control functions of the instrument, some instruments have also had secondary functions such as for outputting the operating status (normal or abnormal) of the instrument itself to the exterior and for outputting pulses.
The primary measurement and control functions are normally implemented continuously, while the secondary functions are operated periodically or non-periodically based on instructions from the CPU, and do not operate at other times. That is, based on instructions from the CPU, the secondary functions are caused to operate only when necessary.
A two-wire field instrument does not have a dedicated power supply line and instead must generate its operating power using a current of 4 to 20 mA sent from a two-wire transmission path. In other words, when measurement values are output, currents of 4 to 20 mA are considered to be within the adjustable range and are used, and therefore the primary measurement and control functions must be implemented using a current of less than 4 mA. Here, if the aforementioned secondary functions are constantly operated, the operation of the primary measurement and control functions will be obstructed. Therefore, the secondary functions are operated only when necessary (for example, see Patent Reference Document 1 (Japanese Patent Laid-Open Publication No. H11-183575)).
FIG. 4 shows the configuration of the main part of a conventional two-wire field instrument. In this figure, reference numeral 100A denotes the field instrument and reference numeral 200A denotes a higher-level device (monitoring device) connected via a two-wire transmission path AL (AL1 and AL2) to the field instrument 100A. In this example, the field instrument 100A is a differential pressure/pressure transmitter.
The field instrument 100A is comprised of a sensor unit 1A that receives pressure and differential pressure and generates an analog signal according to the magnitude thereof, an A/D converter 2A that converts the analog signal from the sensor unit 1A into a digital signal, a CPU 3A that samples the digital signal output from the A/D converter 2A and calculates the measurement value of the pressure/differential pressure, a D/A converter 4A that converts the digital measurement value calculated by the CPU 3A into a corresponding analog signal having a predetermined current range (4 to 20 mA), a communication unit 5A that outputs the analog signal outputted from the D/A converter 4A to a two-wire transmission path AL, a voltage regulator (first power supply unit) 6A that, based on the 4 to 20 mA of current obtained from the two-wire transmission path AL, generates a stable DC voltage V1 (DC 6.5V) as a first power supply, a voltage regulator 7A that receives a supply of power from the voltage regulator 6A and generates a stable DC voltage V2 (DC 3V) as a second power supply, an alarm driving circuit 8A, a selector switch 9A, a ROM 10A, and a RAM 11A.
In the field instrument 100A, the CPU 3A receives a supply of power from the voltage regulator 7A and operates, and while accessing the RAM 11A, operates according to a program stored in the ROM 10A. The ROM 10A stores a measurement value processing program that samples the digital signal from the A/D converter 2A, calculates the pressure/differential pressure measurement value and notifies a monitoring device 200A and also a selector switch control program that detects abnormalities occurring inside the instrument itself and turns ON the selector switch 9A.
FIG. 4 shows a measurement value calculation unit 3A1 and a selector switch control unit 3A2 as function blocks within the CPU 3A, but the measurement value calculation unit 3A1 and the selector switch control unit 3A2 are realized as processing functions according to the program of the CPU 3A.
Also, in this example, the alarm driving circuit 8 is a contact output circuit comprised of a photocoupler provided with a light emitting part and a light receiving part. In this case, when the selector switch 9A is turned ON, current from the voltage regulator 7A flows as drive current to the light emitting part of the alarm driving circuit (photocoupler) 8A, the light emitting part receives this drive current and emits light that is received by the light receiving unit, and the contact output at the light receiving unit side turns ON to activate an externally attached alarm circuit 300A.
The alarm driving circuit 8A is implemented with a photocoupler for the reasons of ensuring the electrical isolation between input and output, and so as not to generate noise that would affect the processing operations of the CPU 3A.
However, in the field instrument 100A described above, with the alarm driving circuit 8A, a drive current of at least 1 mA is required for the light emitting part of the photocoupler, and therefore if an attempt is made to use a high-performance CPU as the CPU 3, there will be insufficient power to drive the CPU 3A.
Accordingly, with the conventional field instrument 100A, in cases where there was a desire to use a high-performance CPU as the CPU 3A, installation of the alarm driving circuit 8A had to be abandoned. Otherwise, the only alternative was to tolerate a degradation of CPU performance (degradation of performance of primary processing functions) and reduce the power required for the CPU and install the alarm driving circuit 8A.
The present invention was devised to solve this type of problem, and an object of the present invention is to provide a field instrument capable of operating secondary functions only when needed, without degrading the performance of the primary processing functions.